Lithos 149 (2012) 136–145
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Recognizing juvenile and relict lithospheric mantle beneath the North China Craton: Combined analysis of H2O, major and trace elements and Sr–Nd isotope compositions of clinopyroxenes Yantao Hao, Qunke Xia ⁎, Shaochen Liu, Min Feng, Yaping Zhang CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, PR China
a r t i c l e
i n f o
Article history: Received 5 August 2011 Accepted 2 March 2012 Available online 13 March 2012 Keywords: H2O contents Sr–Nd isotope composition Peridotite xenolith North China Craton
a b s t r a c t Recognizing juvenile and relict lithospheric mantle is crucial in unraveling the mechanism of lithospheric thinning (delamination vs. thermal/mechanic erosion). The H2O contents and Sr–Nd isotopic compositions of juvenile lithospheric mantle are expected to be similar to the MORB source (asthenospheric mantle), whereas those of the relict lithospheric mantle should be of typical cratonic character. Consequently, combined analysis of H2O contents and Sr–Nd isotope compositions could be an effective way to distinguish the juvenile and relict lithospheric mantle. Among the peridotite minerals, clinopyroxene is the major host for rare earth elements as well as H2O contents, making it the most suitable target sample for such analyses. We collected fresh peridotite xenoliths hosted by Cenozoic basalts from Beiyan, Shandong province and Yangyuan, Hebei province to carry out combined analyses of major elements, trace elements, Sr–Nd isotopes and H2O contents for clinopyroxene. At both Beiyan and Yangyuan, pyroxene from peridotite xenoliths shows homogenous H2O contents within individual grains, and equilibrium distribution of H2O contents between clinopyroxene and orthopyroxene has been achieved. There is a positive correlation between H2O contents and Al contents in clinopyroxene and orthopyroxene, these features imply that the pyroxenes largely preserve the H2O contents of their mantle source. The variations of H2O contents in clinopyroxene are controlled by partial melting rather than the later episode of mantle metasomatism, because there is a correlation between H2O contents of clinopyroxene and degree of partial melting index (Yb content of clinopyroxene and Mg# of olivine). Based on the correlation between H2O contents and Sr isotope ratios of clinopyroxene, the estimated H2O contents and 87Sr/86Sr ratios of mantle source of peridotites in Beiyan (450 to less than 600 ppm, and ~ 0.7028 respectively) are similar to the MORB source, thereby implying that the lithospheric mantle beneath Beiyan is juvenile. In contrast, the variation of H2O contents and Sr–Nd isotope compositions of clinopyroxene from the Yangyuan peridotites is best explained as relict mantle (H2O contents less than 300 ppm and EM1-type Sr–Nd isotope ratios) coexisting with juvenile lithospheric mantle (H2O contents more than 600 ppm and 87Sr/86Sr about 0.7030). These conclusions are in agreement with previous studies which have demonstrated that the lithospheric mantle beneath Beiyan is made up of juvenile material accreted from the asthenosphere after the North China Craton had undergone thinning. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The North China Craton (NCC) is one of the major cratons in eastern Eurasia with crustal remnants older than 3.8 Ga (Liu et al., 1992). The eastern NCC experienced widespread lithospheric extension and dramatic changes of mantle characteristics. A thick (∼200 km), cold (∼40 mW/m2) and highly refractory lithospheric mantle was in place until the mid-Ordovician, but was replaced by a hot (60–80 mW/m2), thin (80–60 km) and fertile lithospheric mantle during the late Mesozoic ⁎ Corresponding author at: No. 96, Jinzhai Road, 230026, Hefei, Anhui, PR China. Tel.: + 86 551 3607008; fax: + 86 551 3607386. E-mail address:
[email protected] (Q. Xia). 0024-4937/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2012.03.013
period (Menzies et al., 2007, and references therein). The timing, spatial and temporal variation as well as mechanism for lithospheric thinning are hotly debated. Delineating the juvenile (e.g., newly accreted materials from upwelling asthenosphere after the NCC thinning) versus relict lithospheric mantle is crucial to unravel the mechanism of lithospheric thinning (delamination vs. thermal/mechanic erosion). The H2O contents and Sr–Nd isotopic compositions of the juvenile lithospheric mantle are expected to be similar to Mid-Ocean Ridge Basalt (MORB) source, whereas the relict lithospheric mantle should be more like typical cratonic mantle. Therefore, combined analyses of H2O contents and Sr–Nd isotope compositions of mantle-derived xenoliths could be an effective way to distinguish between the juvenile and relict lithospheric mantle.
Y. Hao et al. / Lithos 149 (2012) 136–145
Peridotite xenoliths hosted by alkali basalts are direct samples of the continental lithospheric mantle. They largely preserve the geochemical signatures of the mantle source due to the rapid ascent to the surface (generally within a maximum of 50 h after their entrainment in the host magma, O'Reilly and Griffin, 2010) and “quench” effect response to sudden temperature decrease at the surface. The H2O contents in mantle-derived peridotites may provide information about the distribution of water in the lithospheric mantle. Although the main minerals of peridotite xenoliths, olivine, orthopyroxene and clinopyroxene are nominally anhydrous, they still contain a significant amount of water in crystal defects (Bell and Rossman, 1992). Among the minerals of anhydrous peridotite, clinopyroxene is the main host mineral for rare earth element and also has the highest H2O contents (Bonadiman et al., 2009; Grant et al., 2007; Li et al., 2008; Peslier et al., 2002; Xia et al., 2010; Yang et al., 2008), making it the most suitable mineral for such combined H2O and elemental analyses. Petrological, geochemical and geochronological studies of peridotite xenoliths have shown that the Cenozoic lithospheric mantle beneath Beiyan, Shandong province is made up of newly accreted material from the asthenosphere after the NCC had undergone thinning (Xiao et al., 2010). However relict lithospheric mantle still exists beneath Yangyuan, Hebei province (Xu et al., 2008). Furthermore, geophysical observations have detected mantle upwelling beneath Yangyuan, implying that juvenile mantle may be present (Zhao and Xue, 2010). We selected peridotite xenoliths hosted by Cenozoic basalts from Beiyan and Yangyuan for analyses of major and trace elements, Sr–Nd isotopes and H2O contents of clinopyroxene, to verify whether such combined analyses can distinguish between juvenile and relict lithospheric mantle. 2. Geological background and sample petrology The NCC is separated from the Mongolian Block by the eastern Central Asian Orogenic Belt in the north, and from the Yangtze block (part of the South China Block) by the Triassic Dabie Shan– Sulu UHP belt in the south and east (Fig. 1). The NCC is crosscut by two large-scale geophysical and geological linear zones. In the east, it is traversed by the Tan-Lu Fault, which is associated with significant Cenozoic and Mesozoic volcanism. In the west, it is cut by the NS trending Daxing'anling–Taihangshan Gravity Lineament, which separates two topographically and tectonically different regions. The region lying west of the Daxing'anling–Taihangshan Gravity Lineament is characterized by thick crust, low heat flow and a thick (>150 km) lithosphere. In contrast, the region in the eastern NCC is characterized by thin crust, high heat flow and thin lithosphere (Fig. 1), which probably
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related to the diachronous lithospheric thinning of the craton (Ma, 1989; Xu, 2007). Samples in this study come from Beiyan, Shandong province and Yangyuan, Hebei province. Beiyan is located east of Tan-Lu Fault, at about 5 km southwest of Changle County. Beiyan volcano is one of the Changle–Linqu volcanic fields that lie within the Tan-Lu Fault (Zheng et al., 1998). The Beiyan xenoliths analyzed in this paper are the same as those referred by Xiao et al. (2010), where their detailed petrology is given. Yangyuan is located in the western part of NCC. At Yangyuan, spinel peridotites and subordinate pyroxenites xenoliths are hosted by Cenozoic alkali basalts (39–32 Ma K–Ar age; Wang et al., 1989). Yangyuan peridotite xenoliths are fresh, consisting of spinel lherzolites and a few harzburgites. Their textures vary from protogranular to porphyroclastic (Mercier and Nicolas, 1975). Olivine and orthopyroxene are 5–8 mm in size, while clinopyroxene and spinel are smaller (1–3 mm). Clinopyroxene and spinel occur in direct contact with the large orthopyroxene and olivine grains and generally exhibit triple junction texture, but spinel also commonly forms vermicular crystals inside orthopyroxene or between orthopyroxene and clinopyroxene grains. The typical modal mineral assemblage of Yangyuan peridotites is olivine (50–81 vol.%), orthopyroxene (20–34 vol.%), clinopyroxene (1.4–16.7 vol.%) and spinel (0.8–3.8 vol.%). No hydrous phases or metasomatic secondary phases are observed. 3. Analytical methods The analytical methods for major and trace element concentrations and Sr–Nd isotopic compositions of the Beiyan peridotite xenoliths are given in Xiao et al. (2010). In the present paper, H2O contents for clinopyroxene and orthopyroxene were obtained for the same samples by Fourier transform infrared (FTIR) analysis. The analytical methods used for the Yangyuan peridotites include: 3.1. Electron microprobe analyses (EMPA) The mineral compositions were determined using a Shimadzu Electron Probe Micro Analyzer (EMPA 1600) at the CAS Key Laboratory of Crust–Mantle Materials and Environments, the University of Science and Technology of China (USTC), Hefei, with the following operating conditions: 15 kV accelerating voltage, 20 nA beam current and b5 μm beam diameter. Natural minerals and synthetic oxides were used as standards, and a program based on the ZAF procedure was used for data correction. Measurements were made from core to rim of each mineral grain; and in general three to four grains of each mineral were analyzed in each sample. The error for all elements is below 5%, except for Na, which may be up to 10%. 3.2. Laser ablation inductively coupled plasma-mass spectrometry (LA-ICPMS)
Fig. 1. Main geological units of North China Craton (NCC) and sample locations.
Trace element compositions of clinopyroxene were determined at the LA-ICPMS laboratory of USTC. Fresh and clear clinopyroxene grains were selected and mounted in epoxy pellet and polished. Mineral grains were ablated in situ with Coherent company GeoLas pro ArF laser system with beam wavelength 193 nm at 10 Hz repetition rate and 10 J/cm 2 energy per plus. The ablation crater diameters were 60 μm, and the sample aerosol was carried to ICPMS by high purity helium with flow rate of 0.3 L/min. A typical analysis consists of 80–100 replicates Perkin Elmer DRCII ICPMS was used to analyze the aerosol samples with the RF power 1350 W and nebulized gas flow rate 0.7 L/min. Results of the sample analyses were processed with LaTEcalc software. The signal intensities (counts per ppm) for each element were calibrated against the NIST 610 silicate glass and the 44Ca content of the analyzed clinopyroxene was used as an internal standard. Typical analytical precisions ranged from 2% to 5%.
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3.3. Fourier transform infrared spectroscopy (FTIR) Doubly-polished thin sections with thicknesses ranging from 0.2 to 0.4 mm were prepared for FTIR analysis. Unpolarized spectra were obtained from 1000 to 5000 cm − 1 on a Nicolet 5700 FTIR spectrometer coupled with a Continuum microscope at the USTC, using a KBr beam splitter and a liquid-nitrogen cooled MCT-A detector. For clinopyroxene and orthopyroxene, a total of 128 or 256 scans were accumulated for each spectrum at a 2 or 4 cm − 1 resolution. The aperture size was set from 30 × 30 to 100 × 100 μm, depending on the size and quality of the mineral grains to be analyzed. Measurements were made on optically clean, inclusion- and crack-free areas (usually the core region of selected grains) under a continuous dry air flush. The detailed FTIR profile analyses performed at the USTC lab of two augite megacrysts hosted by Nushan Cenozoic basanites confirmed the homogeneity of their H2O contents. These augites were used as standards to detect potential instrument drift during analysis. During the analyses of the Beiyan and Yangyuan peridotites, the maximum variation for the two augites was b4% for both of peak height and the integrated area within the OH absorption area. 3.4. Thermal ionization mass spectrometer (TIMS) The clinopyroxene separates were crushed in a steel-jaw crusher and powdered in an agate mill to a grain size of b200 mesh for Sr– Nd analyses. The pre-chemical procedures and TIMS (multi-collector Finnigan MAT-262 mass spectrometer) were carried out at the USTC. The mass fractionation corrections for Sr and Nd isotope ratios were based on 86Sr/ 88Sr = 0.1194 and 146Nd/ 144Nd = 0.7219. Repeat analyses yielded 87Sr/ 86Sr of 0.70266 ± 0.000015 for the NBS-987 standard and 143Nd/ 144Nd of 0.511849 ± 0.000011 for the La Jolla standard. 4. Results In this section we describe the results obtained for the Yangyuan samples and only the FTIR results for the Beiyan samples. 4.1. Mineral chemistry and thermometry The EMP analyses have shown that the minerals of the Yangyuan peridotites are homogenous (i.e. no chemical variation are observed within a grain or among grains of the same sample). The Mg# (Mg# = 100 × Mg / (Mg + Fe), mol% of ol, orthopyroxene and clinopyroxene from Yangyuan peridotites range from 88.3 to 90.8, 89.0 to 91.3, and 89.4 to 92.1, respectively; the Cr# (Cr# = 100 × Cr / (Cr + Al), mol% in spinel for all xenoliths vary from 7.3 to 44.3 (see Supplementary material). Major elements (Mg, Al, Ca and Cr) show good correlations with Mg# for ol, orthopyroxene and clinopyroxene (not shown). Both experimental studies (e.g., Jaques and Green, 1980) and natural samples (e.g., Arai, 1994) show that Mg# of olivine and Cr# of spinel progressively increase in peridotite residues with increasing melt extraction. Mg# of olivine and Cr# of spinel of the Yangyuan samples fall within olivine-spinel mantle array (not shown), suggesting that different degrees of partial melting were responsible for their compositions. Equilibrium temperatures are estimated using the Ca-inorthopyroxene and the two-pyroxene geothermometers of Brey et al. (1990) at a pressure of 15 kbar and range from ~ 900 to 1100 °C. 4.2. Trace elements of clinopyroxene Most of the incompatible trace elements in anhydrous spinel lherzolites are hosted by clinopyroxenes. The trace element contents of
clinopyroxenes from the Yangyuan xenoliths are homogenous within the same sample and the average values are given in Table 1. Fig. 2a and b shows rare earth element (REE) and trace element patterns for the Yangyuan clinopyroxene, which are normalized to C1 chondrite values (Sun and McDonough, 1989, identified with “n” subscript) and primitive mantle values (McDonough and Sun, 1995, identified with “N” subscript) respectively. The REE patterns can be grouped into three categories: (1) Light rare earth element (LREE) depleted patterns, (2) Flat REE patterns, and (3) LREE-enriched patterns. The LREE-depleted patterns are characterized by depleted LREE and a flat HREE pattern, with (La/Sm)n = 0.38–0.60 and (Ho/ Yb)n = 1.06–1.19 and have a well developed negative Ti anomalies (Ti* = 2 × Ti/(Eu + Gd)N, range from 0.37 to 0.46) and a weak Zr (and Hf) anomalies (Zr* = 2 × Zr/(Nd + Sm)N, range from 0.54 to 0.83). The flat REE patterns are characterized by a flat LREE pattern ((La/Sm)n = 1.15–3.05) as well as a flat HREE pattern ((Ho/Yb)n = 1.1–1.2). The LREE-enriched patterns are characterized by LREE enrichments ((La/Sm)n = 1.01–9.52) and fractionated HREE patterns ((Ho/Yb)n = 1.20–1.95) and have well developed Zr, Hf and Ti strong negative anomalies (Zr* = 0.32–0.66, Ti* = 0.03–0.38) and enrichments of Th and U. 4.3. Sr–Nd isotopes 87
Sr/ 86Sr ratios of clinopyroxene in the Yangyuan peridotites ranges from 0.7028 to 0.7041 and 143Nd/ 144Nd ratios vary from 0.5127 to 0.5134 (Table 2). In the Sr–Nd plot (Fig. 3), they display a good negative correlation. Combine with the data for Yangyuan peridotites reported by Xu et al. (2008), the high 87Sr/ 86Sr ratio and medium 143Nd/ 144Nd ratio of some samples point to a contribution from an EMI-type mantle component. 4.4. Hydrogen speciation and H2O contents All the analyzed pyroxene grains in these peridotite xenoliths have several absorption bands in the OH-stretching vibration region (2800–3800 cm − 1). Representative infrared spectra for clinopyroxene and orthopyroxene from Beiyan and Yangyuan peridotites are shown in Fig. 4. The IR absorption bands of pyroxenes can be subdivided into three groups: for clinopyroxene: 3630–3620 cm − 1, 3540–3520 cm − 1, 3470–3450 cm − 1; and for orthopyroxene: 3600–3580 cm − 1, 3520–3510 cm − 1, 3420–3410 cm − 1. The positions of these absorption bands are similar to those reported in earlier studies, and are interpreted as resulting from the vibration of structural OH (Bell and Rossman, 1992; Bonadiman et al., 2009; Gose et al., 2009; Grant et al., 2007; Ingrin and Skogby, 2000; Li et al., 2008; Peslier et al., 2002; Skogby and Rossman, 1989; Skogby et al., 1990; Xia et al., 2010; Yang et al., 2008). The relative absorbance of these bands varies among grains in a given sample due to variable orientation of grains with respect to the IR beam direction. Measurement of hydrogen profiles performed on the larger pyroxene grains in each suite of samples show no obvious variations in absorbance areas between core and rim regions (not shown). The infrared spectra of the Beiyan and Yangyuan peridotites are similar. A modified form of the Beer–Lambert law was used to calculate the H2O contents: c ¼ A=ðI t Þ where c is the content of hydrogen species (ppm H2O wt.%), A is the integrated area (cm − 2) of absorption bands in the region of interest, I is the integral specific absorption coefficient (ppm − 1 cm − 2), and t is thickness (cm). OH absorption bands were integrated between 3000 and 3800 cm− 1 for clinopyroxene and 2800 to 3800 cm− 1 for orthopyroxene and multiplied by 3 to obtain A values (Kovacs et al., 2008); the integral specific coefficients of 7.09 ppm− 1 cm− 2 for clinopyroxene
Table 1 Trace element concentration (ppm) of clinopyroxenes from Yangyuan peridotite xenoliths. YY-3-2
±
YY-3-3
±
YY-3-4
±
YY-3-6
±
YY-3-9
±
YY-3-10
±
YY-3-12
±
YY-3-13
±
YY-3-15
±
YY-3-17
±
YY-3-21
±
Sc Ti V Co Ni Ga Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U
71.7 1751 267 18.9 330 3.29 0.00 68 15.0 28.5 0.07 0.000 2.83 4.97 0.66 3.66 1.49 0.58 2.13 0.387 2.71 0.592 1.696 0.236 1.488 0.212 1.023 0.020 0.636 1.851 0.577
5.2 127 13 0.6 15 0.22 0.01 3 0.6 1.1 0.03 0.032 0.54 0.77 0.05 0.24 0.15 0.01 0.09 0.016 0.22 0.004 0.179 0.014 0.132 0.016 0.037 0.007 0.051 0.523 0.180
63.9 1941 273 21.9 359 3.80 n.d. 75 14.3 24.3 0.15 0.000 1.19 4.64 0.87 5.23 1.75 0.64 2.22 0.383 2.54 0.567 1.572 0.219 1.422 0.199 0.823 0.015 0.083 0.015 0.003
1.1 20 4 0.1 10 0.26
75.2 951 242 27.4 457 3.01 0.01 354 5.2 12.2 3.96 1.096 18.05 21.81 1.66 5.38 1.22 0.44 1.35 0.197 1.07 0.215 0.537 0.072 0.488 0.073 0.427 0.071 0.754 4.211 0.764
6.5 86 15 9.6 140 0.37 0.03 136 0.9 10.6 6.11 2.086 5.80 5.80 0.42 1.30 0.05 0.05 0.27 0.027 0.14 0.030 0.091 0.014 0.124 0.013 0.242 0.115 1.019 5.782 1.028
64.6 2180 289 20.8 327 4.54 0.01 143 13.9 25.0 0.14 0.159 7.56 17.19 2.10 9.11 2.13 0.80 2.35 0.419 2.59 0.554 1.497 0.209 1.380 0.196 0.944 0.006 0.452 0.790 0.207
4.3 59 28 0.4 2 1.06 0.01 193 1.6 7.8 0.14 0.284 13.97 29.21 2.92 10.08 1.27 0.35 0.48 0.059 0.45 0.055 0.166 0.018 0.123 0.020 0.184 0.006 0.166 1.231 0.247
54.3 1434 246 19.9 320 3.92 n.d. 138 12.3 12.0 0.07 0.000 4.95 9.07 0.93 3.80 1.05 0.49 1.53 0.303 2.22 0.500 1.415 0.200 1.355 0.194 0.382 0.010 0.543 0.557 0.136
2.3 102 0 0.3 5 0.22
62.0 1801 260 19.3 336 3.67 0.00 63 16.1 20.5 0.09 0.001 1.36 3.97 0.61 3.90 1.52 0.62 2.16 0.402 2.77 0.611 1.734 0.242 1.716 0.239 0.719 0.036 0.098 0.034 0.013
3.7 32 10 1.4 18 0.53 0.02 2 1.0 0.4 0.10 0.019 0.03 0.24 0.05 0.37 0.07 0.06 0.07 0.034 0.24 0.069 0.056 0.033 0.054 0.027 0.055 0.002 0.046 0.013 0.019
63.7 1589 257 22.7 384 3.83 0.01 88 14.7 24.4 0.52 0.143 2.88 8.03 1.15 5.37 1.62 0.63 2.20 0.401 2.75 0.577 1.627 0.223 1.407 0.204 0.814 0.020 0.138 0.092 0.031
2.7 33 4 0.5 7 0.22 0.03 2 0.5 0.7 0.02 0.057 0.08 0.12 0.03 0.21 0.06 0.05 0.11 0.013 0.11 0.018 0.080 0.013 0.067 0.011 0.011 0.012 0.055 0.007 0.015
61.6 1516 247 21.6 380 3.34 n.d. 45 14.1 18.7 0.38 0.031 0.91 2.75 0.48 3.01 1.31 0.48 1.97 0.367 2.60 0.558 1.579 0.209 1.471 0.199 0.749 0.043 0.089 0.046 0.010
1.5 86 9 1.2 15 0.13
72.0 486 232 20.7 346 7.37 0.01 376 20.1 143.7 0.54 0.000 12.05 34.46 5.27 28.01 7.70 2.62 7.03 0.969 4.82 0.801 1.778 0.223 1.227 0.160 2.192 0.048 0.391 0.612 0.146
0.8 24 14 0.3 8 0.27 0.01 22 1.7 14.8 0.05 0.000 0.69 1.76 0.23 1.09 0.70 0.22 0.63 0.068 0.24 0.047 0.052 0.016 0.056 0.006 0.383 0.003 0.082 0.083 0.017
62.6 2151 252 22.2 375 3.32 n.d. 55 14.5 25.0 0.23 0.028 0.78 2.64 0.49 3.22 1.28 0.60 2.06 0.356 2.55 0.574 1.590 0.220 1.481 0.207 0.760 0.018 0.042 0.023 0.006
3.7 85 9 0.5 9 0.18
68.4 517 247 22.2 383 6.47 0.04 318 22.9 139.4 1.23 0.289 14.91 44.12 6.53 32.33 7.55 2.40 6.68 0.920 5.01 0.897 2.229 0.276 1.743 0.228 1.075 0.147 0.274 0.262 0.061
1.7 18 9 1.3 9 0.68 0.06 18 1.4 27.6 0.04 0.586 0.20 1.05 0.15 2.07 0.45 0.12 0.27 0.095 0.45 0.067 0.200 0.024 0.092 0.004 0.179 0.032 0.027 0.021 0.011
2 0.1 1.0 0.03 0.000 0.06 0.18 0.03 0.39 0.14 0.04 0.03 0.018 0.06 0.009 0.068 0.006 0.020 0.007 0.088 0.008 0.045 0.002 0.004
40 0.6 0.0 0.02 0.071 1.58 2.02 0.40 1.56 0.17 0.03 0.05 0.004 0.14 0.028 0.048 0.003 0.041 0.003 0.070 0.002 0.046 0.081 0.011
1 0.9 1.3 0.02 0.063 0.05 0.12 0.02 0.32 0.08 0.07 0.21 0.008 0.18 0.018 0.103 0.016 0.024 0.007 0.040 0.005 0.053 0.018 0.005
2 0.9 1.5 0.02 0.025 0.10 0.13 0.04 0.09 0.15 0.02 0.11 0.021 0.15 0.021 0.069 0.013 0.060 0.008 0.012 0.006 0.014 0.007 0.010
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Samples
Note: n.d. = not detected.
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Fig. 2. A, B REE and trace element patterns of clinopyroxene from Yangyuan peridotites. REE are normalized to C1 chondrite values (Sun and McDonough, 1989) and identified with “n” subscript, trace elements are normalized to primitive mantle values (McDonough and Sun, 1995) and identified with “N” subscript.
Table 2 H2O contents and Sr–Nd isotope composition of Yangyuan and Beiyan peridotites. Samples
YY-3-2 YY-3-3 YY-3-4 YY-3-6 YY-3-9 YY-3-10 YY-3-11 YY-3-12 YY-3-13 YY-3-15 YY-3-17 YY-3-21 CLB05-07 CLB05-22 CLB05-25 CLB05-30 CLB05-31 CLB05-01 CLB05-46
Rock type
Water content(ppm) cpx
opx
lh lh hz hz lh lh lh lh lh lh lh lh lh lh lh lh lh weh weh
524 411 258 336 654 432 465 332 364 342 404 413 226 156 453 291 77
177 200 129 136 220 178 225 199 164 149 163 155 41 55 114 77 15 35 60
εNd
87Sr/86Sr
± 2σ
143Nd/144Nd
± 2σ
0.704116 0.703028
22 19
0.512834 0.513372
18 10
0.38 1.43
0.703640 0.704077 0.703600 0.703471 0.7 0.7 0.7 0.7 0.7 0.703260 0.703180
24 20 23 20 13 12 13 13 13 13 12
0.513290 0.513034 0.513822 0.513096 0.51 0.51 0.51 0.51 0.51 0.51 0.51
8 9 9 6 12 13 13 13 7 13 13
1.27 0.77 2.31 0.89 6.55 4.99 7.75 5.94 14.7 6.05 5.64
Notes: (1) Sr and Nd isotope date of the Beiyan peridotite are from Xiao et al. (2010). (2) Rock type. lh: lherzolite, hz: harzburgite, weh: wehrlite.
Y. Hao et al. / Lithos 149 (2012) 136–145
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5. Discussion 5.1. Depletion and enrichment events
Fig. 3. Sr–Nd isotope composition of clinopyroxene from Yangyuan peridotites.
and of 14.84 ppm− 1 cm− 2 for orthopyroxene were used (Bell et al., 1995) to calculate H2O contents; thickness was measured using a digital micrometer and reported as the average of 30–40 measurements covering the whole section. Baseline corrections were carried out by hand, at least three times for each spectrum, the uncertainty was less than 5%, and the average corrected spectrum was used to calculate H2O contents. To minimize possible uncertainties from the unpolarized determination of these optically anisotropic minerals, more than 15–20 different grains of each mineral in the same sample were analyzed, and the average value was used to define the H2O contents of the mineral in that sample (Asimow et al., 2006; Grant et al., 2007; Kovacs et al., 2008). Uncertainties in the calculated H2O contents come from (1) using unpolarized infrared beams on unoriented minerals (b10%, Kovacs et al., 2008); (2) baseline correction (b5%); (3) variable sample thickness (b3%); and (4) differences between the absorption coefficients (b10%) of our samples and those of samples used by Bell et al. (1995) due to differences in composition. The total uncertainty is estimated to be less than 20–30%. The H2O contents measured in clinopyroxene and orthopyroxene are given in Table 2. H2O contents in the Beiyan peridotites vary from 77 ppm to 453 ppm for clinopyroxene and from 15 ppm to 114 ppm for orthopyroxene; those in the Yangyuan peridotites vary from 258 ppm to 654 ppm for clinopyroxene and from 129 ppm to 225 ppm for orthopyroxene.
Peridotites usually are the residue of mantle partial melting after basaltic component extraction, as evidenced by the relationship between Mg# of olivine and Cr# of spinel for the Yangyuan peridotites. Following the method of Norman (1998), the degree of partial melting for the Yangyuan peridotite xenoliths is estimated to be less than 10%, based on the correlation between YN and YbN (Fig. 5). Samples YY-3-15 and YY-3-21 deviate from the model melting curve and may be induced by metasomatism. Although no hydrous minerals are found in the Yangyuan peridotite xenoliths, the large variations of (La/Yb) ratio and LREE and LILE (Large Ion Lithophile Elements) enrichment of clinopyroxene (Fig. 2) indicate that these peridotites had undergone cryptic mantle metasomatism (Dawson, 1984). Chromatographic-type migration of LREE enriched melts/fluids through LREE-depleted peridotites is an efficient metasomatic mechanism (Bodinier et al., 1990; Navon and Stolper, 1987) to produce various REE patterns, and the variations are largely controlled by the initial degrees of depletion and the extent of the subsequent melt/fluid-driven reactions (Ionov et al., 2002). This is typical of that observed in the Yangyuan clinopyroxene with LREE-enriched patterns (Fig. 2). Coltorti et al. (1999) made a comparison of clinopyroxene trace element contents of a large set of metasomatized xenoliths, ranging from proposed alkali-silicic metasomatism to carbonatitic metasomatism; the Ti/Eu versus (La/Yb)n ratios may be taken as indicators of carbonatitic metasomatism which usually has Ti/Eu b 1500, and (La/Yb)n >3–4 (see Fig. 14 in Coltorti et al., 1999). Most of the Yangyuan samples have high Ti/Eu and low (La/Yb)n ratios, indicating that the main metasomatic agent was silicate melt. Samples of YY-3-15 and YY-3-21 have low Ti/Eu and (La/Yb)n > 5, which indicate carbonatitic metasomatism. In addition, the sample YY-3-4 with (La/Yb)n > 25 and Ti/Eu > 2000 may have experienced both silicate and carbonatitic metasomatism (Fig. 6). The carbonatitic metasomatism for YY-3-15 and YY-3-21 may be responsible for the deviation of Y and Yb in the model melting curve (Fig. 5). The major and trace element data has led Xiao et al. (2010) to infer similar depletion and enrichment events for the Beiyan peridotites, and they suggested that the metasomatic agents was mainly derived from silicate melts from an upwelling asthenosphere based on depleted Sr–Nd isotopic compositions.
Fig. 4. Representative IR spectra of pyroxene in Yangyuan and Beiyan peridotites.
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Fig. 5. Comparison of Y and Yb contents of clinopyroxenes in peridotite xenoliths from Yangyuan. Using fractional melting model within spinel stability field of Norman, 1998. The subscript “N” indicates that the Y and Yb concentrations have been normalized to primitive mantle compositions (McDonough and Sun, 1995). Dcpx = 0.42 for Y and 0.40 for Yb, and Xcpx = 0.2.
2009; Grant et al., 2007; Peslier et al., 2002; Xia et al., 2010; Yang et al., 2008), perhaps because of the diffusion rates (Demouchy et al., 2006). The pyroxenes of Beiyan and Yangyuan peridotites appear to have largely preserved the initial H2O contents of the mantle source based on the following observations: (1) Core-rim profile analyses for clinopyroxene and orthopyroxene grains from the Beiyan and Yangyuan peridotites do not reveal H2O heterogeneities. In contrast, the typical character of hydrogen loss is heterogeneous distribution of OH content within mineral grain (higher in the core and lower at the rim, Demouchy et al., 2006; Peslier and Luhr, 2006). (2) H2O contents between clinopyroxene and orthopyroxene are positively correlated. Xia et al. (2010) collected about 150 data of H2O contents in pyroxenes of peridotites from Cenozoic basalts in the NCC and found that the partition coefficient between clinopyroxene and orthopyroxene is 2.2 ± 0.5. The partition coefficient between clinopyroxene and orthopyroxene for the Beiyan and Yangyuan peridotites is ~ 2.45 (Fig. 7). These partition coefficients are in agreement with those for the other NCC peridotites, and are also in the range (2.3 ± 0.5) of those found in natural peridotite xenoliths worldwide (Bell and Rossman, 1992; Grant et al., 2007; Li et al., 2008; Peslier et al., 2002).
5.2. Preservation of initial H2O contents of the mantle source 5.3. Recognizing juvenile and relict lithospheric mantle In low pressure condition (P b 3.5 GPa), the solubility of hydrogen in nominally anhydrous minerals decrease with decreasing pressure (Keppler and Bolfan-Casanova, 2006; Mierdel et al., 2007); thus, when peridotite xenoliths are brought to the surface by their host magmas, hydrogen can potentially diffuse out of the nominally anhydrous minerals. Experiments (Hercule and Ingrin, 1999; Kohlstedt and Mackwell, 1998; Stalder and Skogby, 2003; Woods et al., 2000) have shown that at 1000 °C, hydrogen resetting in olivine and pyroxene will be achieved at millimeter scales quickly, in a few tens of hours. In contrast, analyses of natural samples suggest that pyroxenes preserve their mantle derived OH contents, whereas olivines do not (Bell and Rossman, 1992; Bell et al., 2004; Gose et al., 2009; Grant et al., 2007; Peslier et al., 2002; Xia et al., 2010). Possible explanations for the discrepancy between experimental results and observations on natural samples may be related to: (1) the loss of hydrogen in minerals is influenced by the surrounding conditions, such as H2O contents of coexisting minerals and melts, H2O contents and oxygen fugacities of the systems; (2) the incorporation of hydrogen into minerals does not depend only on the diffusion rate of hydrogen, but also on the diffusion rate of point defects associated with hydrogen incorporation, which is comparatively slower by at least several orders of magnitude (Kohlstedt and Mackwell, 1998); and (3) experiments are made under water-saturated conditions, whereas these conditions are seldom achieved in the mantle. The loss of hydrogen in olivine (Demouchy et al., 2006; Peslier and Luhr, 2006; Peslier et al., 2008) contrast with the preservation of hydrogen in pyroxene (Gose et al.,
Fig. 6. Plot of (La/Yb)n vs. Ti/Eu of clinopyroxene from Yangyuan peridotites. Fields for “carbonatitic” and “silicate” metasomatism are after Coltorti et al. (1999).
5.3.1. Beiyan region Yb is a HREE and is relatively immobile during mantle metasomatism. Therefore, Yb contents are considered to represent robust indicators of degree of partial melting. The positive correlation between H2O and Yb contents in clinopyroxene of the Beiyan peridotites (Fig. 8a), suggests that the variation of the H2O contents was largely controlled by partial melting rather than metasomatism. That the H2O contents of the Beiyan peridotite clinopyroxene are not related to metasomatism is further corroborated by the lack of correlation between H2O contents and (La/Yb)n ratios, since the (La/Yb)n ratios of clinopyroxene is a good indicator of degree of mantle metasomatism (Fig. 8b). 87 Sr/86Sr ratios are negatively correlated with YbN in clinopyroxene (Fig. 9a) and the H2O contents in both clinopyroxene and orthopyroxene (Fig. 9b). These correlations are related to mantle metasomatism, as proposed in Fig. 10. During partial melting process, H2O contents, (Yb)N and Rb/Sr ratios in clinopyroxene decrease with increasing degree of melting: i.e., points 1 to 5 in Fig. 10 represent 5 samples that experienced different degrees of melting. Points 1 to 5 would evolve to different 87Sr/86Sr ratios by radiogenic processes because of their different initial Rb/Sr ratios, resulting in highest 87Sr/ 86Sr ratio for point 1 and lowest 87Sr/86Sr ratio for point 5. Accordingly we should expect positive correlations between 87Sr/86Sr ratios and H2O and YbN values. However, this is not the case for the Beiyan clinopyroxene (Fig. 9a and b). Instead, there is a negative correlation between 87Sr/86Sr ratios and H2O and YbN values. This can be explained by mantle metasomatism which resulted in enriched LREE and LILE patterns (Fig. 2). The samples that had undergone higher degrees of partial melting should be more prone to metasomatism (Bodinier et al., 1990; Navon and Stolper, 1987), and if the metasomatic agent has higher 87Sr/86Sr than the Beiyan clinopyroxene, the negative correlations of Fig. 9 would result. Although the experiments have shown that the diffusion of Sr in clinopyroxene is much slower than water (Cherniak and Dimanov, 2010; Farver, 2010), both the positive correlation of Yb and water content and the lack of correlation between water contents and (La/Yb)n ratios of our samples suggest that the variation of the water contents was largely controlled by partial melting rather than metasomatism (Fig. 8). In addition, the good correlation of La and Sr (R 2 = 0.91, not shown) indicates the addition of Sr by metasomatism. The decoupling of water and LILE contents are also observed for clinopyroxene from Simcoe and Mexican peridotites (Peslier et al., 2002).
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Fig. 7. Correlation of H2O contents between clinopyroxene and orthopyroxene of Yangyuan and Beiyan peridotites. Symbols are the same as Fig. 6.
The initial H2O contents of the mantle source of the Beiyan peridotites can be estimated with the help of Fig. 8a. Assuming that the most primitive Beiyan peridotite had YbN = 5 (Norman, 1998), the H2O contents in the source was 557 ppm. This should be the upper limit of H2O contents because the mantle of Beiyan could not be as primitive as the primitive mantle. The highest H2O contents in clinopyroxene of the Beiyan peridotite is 450 ppm from sample CLB05-25, which should be the lower limit estimation of the Beiyan mantle source. So, the H2O contents of the mantle source of the Beiyan peridotites range between 450 and 557 ppm. Similarly, an 87Sr/ 86Sr ratio of 0.7028 is estimated for the Beiyan mantle source from Fig. 9b. The H2O contents and 87Sr/ 86Sr ratios of clinopyroxene in the MORB source are Fig. 9. A H2O contents of pyroxene vs. Sr isotope composition in clinopyroxene from Beiyan peridotites B. YbN vs. Sr isotope composition in clinopyroxene from Beiyan peridotites.
estimated to be 500 ± 250 ppm (Dixon et al., 2004; Hirth and Kohlstedt, 1996; Peslier et al., 2010) and 0.7025–0.7035 (Zindler and Hart, 1986) respectively; clinopyroxene in the OIB source have H2O contents >1000 ppm (Dixon et al., 2004; Hirth and Kohlstedt, 1996; Peslier et al., 2010) and 87Sr/ 86Sr ratios of 0.703–0.705 (Zindler and Hart, 1986) respectively. Accordingly, the inferred mantle source of the Beiyan peridotites is similar to MORB source. In addition, Beiyan is located close to the Tan-Lu fault that could have facilitated upwelling of asthenosphere material, Therefore, we
Fig. 8. A YbN vs. H2O contents in clinopyroxene from Beiyan peridotites B (La/Yb) N vs. H2O contents in clinopyroxene from Beiyan peridotites.
Fig. 10. Cartoon of Sr isotope composition evolution.
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Fig. 11. Mg# in olivine vs. H2O contents in clinopyroxene from Yangyuan peridotites.
conclude that the lithospheric mantle of Beiyan represented by peridotite xenoliths is juvenile (e.g. newly accreted from asthenospheric mantle) after the NCC thinning. 5.3.2. Yangyuan region As discussed in Section 5.1, the Yangyuan peridotites have undergone not only partial melting, but also metasomatism events. Samples of YY-3-15 and YY-3-21 have fractionated HREE patterns, and may have undergone carbonatitic metasomatism. These two samples do not follow the model melting curve of Y and Yb, thus, we employ the Mg# value of olivine as a melting index instead of YbN in clinopyroxene. As shown in Fig. 11, H2O contents of clinopyroxene and Mg# values of olivine are negatively correlated. In contrast, H2O contents of clinopyroxene do not show systematic variation between the nonmetasomatized, weakly-metasomatized and strongly-metasomatized samples (based on (La/Yb)n ratios). Thus, the main control factor of H2O contents of clinopyroxene is partial melting rather than later mantle metasomatism events. The 87Sr/ 86Sr ratios and H2O contents in clinopyroxene from the Yangyuan peridotites are negatively correlated (Fig. 12). The nonmetasomatized samples (YY-3-13 and YY-3-17, (La/Yb)n b 1) have similar 87 Sr/86Sr ratios and H2O contents as the strongly-metasomatized sample (YY-3-21, (La/Yb)n =6.1), implying that such negative correlations cannot be explained by a single metasomatism event, in contrast with the Beiyan peridotites. A model with two metasomatic events is proposed for the Yangyuan lithospheric mantle: the first event occurred in the ancient lithospheric mantle before the thinning of NCC represented by samples YY-3-6, YY-3-13, YY-3-15 and YY-3-17 with high 87Sr/86Sr ratios (0.7036 to 0.7041) and low H2O contents (336–404 ppm), and the second event occurred in the juvenile lithospheric mantle represented by samples YY-3-9 and YY-3-21 with low 87Sr/86Sr ratios (0.7030 to 0.7036) and high H2O contents (413–654 ppm), sample YY-3-9 has similar H2O content (654 ppm) and 87Sr/86Sr ratio (0.7030) to the MORB source. This model suggests that relict and juvenile lithospheric mantle coexist in Yangyuan region, in agreement with previous geochemistry (Xu et al., 2008) and geophysics (Zhao and Xue, 2010) observations. 6. Conclusions (1) Water has been incorporated in clinopyroxene and orthopyroxene of the Yangyuan and Beiyan peridotites as OH defects. The H2O contents of clinopyroxene and orthopyroxene for the Yangyuan peridotites range from 258 to 654 ppm and 129 to 225 ppm respectively, and those for the Beiyan peridotites range from 77 to 453 ppm and 15 to 114 ppm respectively. The variation of H2O contents of both pyroxenes is mainly controlled by partial melting rather than later mantle metasomatism.
Fig. 12. H2O contents and Sr isotope evolution in clinopyroxene from Yangyuan peridotites. Two metasomatic events occurred in (I) the ancient lithospheric mantle before the thinning of NCC and (II) the juvenile lithospheric mantle. See the text for detailed explanation.
(2) Peridotite xenoliths from Beiyan were strongly affected by mantle metasomatism. The mantle metasomatism event did not change H2O contents but Sr isotope composition. Base on the correlation of H2O contents and 87Sr/ 86Sr ratios of clinopyroxene, the estimated H2O contents and 87Sr/ 86Sr ratios of the mantle source of the Beiyan peridotites (450 to 557 ppm, and ~0.7028 respectively) are the same as the MORB source, suggesting that the lithospheric mantle beneath Beiyan is juvenile (e.g. newly accreted from upwelling asthenosphere) after the NCC thinning. (3) As to Yangyuan peridotites, the variation of H2O contents and Sr-Nd isotope compositions of clinopyroxene can be explained as relict mantle (H2O contents less than 300 ppm and EM1type Sr–Nd isotope ratios) coexisted with juvenile lithospheric mantle (H2O contents more than 600 ppm and 87Sr/ 86Sr about 0.7030). (4) The combination analysis of major and trace elements, Sr–Nd isotopes and H2O contents for clinopyroxene is an effective way to discriminate the relict cratonic and newly accreted lithospheric mantle.
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